# std::ranges::binary_search

From cppreference.com
< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
| (1) |
template< std::forward_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity, std::indirect_strict_weak_order <const T*, std::projected<I, Proj>> Comp = ranges::less > constexpr bool binary_search( I first, S last, const T& value, Comp comp = {}, Proj proj = {} ); |  | (since C++20) (until C++26)
template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, class T = std::projected_value_t<I, Proj>, std::indirect_strict_weak_order <const T*, std::projected<I, Proj>> Comp = ranges::less > constexpr bool binary_search( I first, S last, const T& value, Comp comp = {}, Proj proj = {} ); |  | (since C++26)
| (2) |
template< ranges::forward_range R, class T, class Proj = std::identity, std::indirect_strict_weak_order <const T*, std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less > constexpr bool binary_search( R&& r, const T& value, Comp comp = {}, Proj proj = {} ); |  | (since C++20) (until C++26)
template< ranges::forward_range R, class Proj = std::identity, class T = std::projected_value_t<ranges::iterator_t<R>, Proj>, std::indirect_strict_weak_order <const T*, std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less > constexpr bool binary_search( R&& r, const T& value, Comp comp = {}, Proj proj = {} ); |  | (since C++26)

1) Checks if a projected element equivalent to value appears within the range `[`first`,` last`)`.
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

For `ranges::binary_search` to succeed, the range `[`first`,` last`)` must be at least partially ordered with respect to value, i.e. it must satisfy all of the following requirements:

- partitioned with respect to std::invoke(comp, std::invoke(proj, element), value) (that is, all projected elements for which the expression is true precedes all elements for which the expression is false).
- partitioned with respect to !std::invoke(comp, value, std::invoke(proj, element)).
- for all elements, if std::invoke(comp, std::invoke(proj, element), value) is true then !std::invoke(comp, value, std::invoke(proj, element)) is also true.

A fully-sorted range meets these criteria.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | the range of elements to examine
r | - | the range of elements to examine
value | - | value to compare the elements to
comp | - | comparison function to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

true if an element equal to value is found, false otherwise.

### Complexity

The number of comparisons and projections performed is logarithmic in the distance between first and last (at most log2(last - first) + O(1) comparisons and projections). However, for iterator-sentinel pair that does not model std::random_access_iterator, number of iterator increments is linear.

### Notes

`std::ranges::binary_search` doesn't return an iterator to the found element when an element whose projection equals value is found. If an iterator is desired, std::ranges::lower_bound should be used instead.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_algorithm_default_value_type` | `202403` | (C++26) | List-initialization for algorithms (1,2)

### Possible implementation

    struct binary_search_fn
    {
        template<std::forward_iterator I, std::sentinel_for<I> S,
                 class Proj = std::identity, class T = std::projected_value_t<I, Proj>,
                 std::indirect_strict_weak_order
                     <const T*, std::projected<I, Proj>> Comp = ranges::less>
        constexpr bool operator()(I first, S last, const T& value,
                                  Comp comp = {}, Proj proj = {}) const
        {
            auto x = ranges::lower_bound(first, last, value, comp, proj);
            return (!(x == last) && !(std::invoke(comp, value, std::invoke(proj, *x))));
        }

        template<ranges::forward_range R, class Proj = std::identity,
                 class T = std::projected_value_t<ranges::iterator_t<R>, Proj>,
                 std::indirect_strict_weak_order
                     <const T*, std::projected<ranges::iterator_t<R>,
                                               Proj>> Comp = ranges::less>
        constexpr bool operator()(R&& r, const T& value, Comp comp = {}, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), value,
                           std::move(comp), std::move(proj));
        }
    };

    inline constexpr binary_search_fn binary_search;

### Example

Run this code

    #include <algorithm>
    #include <cassert>
    #include <complex>
    #include <iostream>
    #include <ranges>
    #include <vector>

    int main()
    {
        constexpr static auto haystack = {1, 3, 4, 5, 9};
        static_assert(std::ranges::is_sorted(haystack));

        for (const int needle : std::views::iota(1)
                              | std::views::take(3))
        {
            std::cout << "Searching for " << needle << ": ";
            std::ranges::binary_search(haystack, needle)
                ? std::cout << "found " << needle << '\n'
                : std::cout << "no dice!\n";
        }

        using CD = std::complex<double>;
        std::vector<CD> nums{{1, 1}, {2, 3}, {4, 2}, {4, 3}};
        auto cmpz = [](CD x, CD y){ return abs(x) < abs(y); };
        #ifdef __cpp_lib_algorithm_default_value_type
            assert(std::ranges::binary_search(nums, {4, 2}, cmpz));
        #else
            assert(std::ranges::binary_search(nums, CD{4, 2}, cmpz));
        #endif
    }

Output:

    Searching for 1: found 1
    Searching for 2: no dice!
    Searching for 3: found 3

### See also

ranges::equal_range (C++20) | returns range of elements matching a specific key (algorithm function object)
ranges::lower_bound (C++20) | returns an iterator to the first element not less than the given value (algorithm function object)
ranges::upper_bound (C++20) | returns an iterator to the first element greater than a certain value (algorithm function object)
ranges::containsranges::contains_subrange (C++23)(C++23) | checks if the range contains the given element or subrange (algorithm function object)
binary_search | determines if an element exists in a partially-ordered range (function template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/ranges/binary_search&oldid=171933"